Quick Tips on Quality in Machining (1-3)

Due to the complexity of CNC machining—such as different machine tools, materials, cutting tools, cutting methods, parameter settings, and so on—it takes a relatively long time to reach a certain level of proficiency in CNC machining (whether in machining or programming). The following is a compilation of practical experience—summarized by engineers through long-term hands-on production—regarding CNC machining processes, operations, the selection of common cutting tool parameters, and monitoring during the machining process, which is provided for your reference.

I. Q: How should machining operations be divided?

A: CNC machining operations can generally be divided using the following methods:

(1) The “Tool-Based Sequencing Method” involves dividing operations based on the cutting tools used. All machinable areas on a part are completed using a single tool first. Then, the second and third tools are used to complete the remaining areas they can handle. This reduces the number of tool changes, minimizes idle time, and reduces unnecessary positioning errors.

(2) The “by machining area” method: For parts with extensive machining requirements, the machining areas can be divided into several sections based on structural characteristics, such as internal contours, external contours, curved surfaces, or flat surfaces. Generally, flat surfaces and locating surfaces are machined first, followed by holes; simple geometric shapes are machined before complex ones; and areas with lower precision requirements are machined before those with higher precision requirements.

(3) Sequencing by Roughing and Finishing: For parts prone to machining deformation, deformation may occur after roughing that requires reshaping; therefore, in general, operations involving both roughing and finishing should be separated.

In summary, when dividing machining operations, one must flexibly adapt to the part’s structure and manufacturability, the machine tool’s capabilities, the extent of CNC machining required, the number of setups, and the production organization of the facility. Additionally, whether to adopt the principle of process concentration or process dispersion should be determined based on actual conditions, but the approach must always be reasonable.

II. Question: What principles should be followed when arranging the machining sequence?

Answer: The arrangement of the machining sequence should be based on the part’s structure and the condition of the blank, as well as the requirements for positioning and clamping, with a focus on ensuring that the workpiece’s rigidity is not compromised. The sequence should generally follow the principles below:

(1) Machining in the preceding operation must not interfere with the positioning and clamping of the subsequent operation; operations involving general-purpose machine tools interspersed between them must also be taken into comprehensive consideration.

(2) Internal shape and cavity machining operations should be performed first, followed by external shape machining operations.

(3) Operations that use the same positioning and clamping methods or the same cutting tool should ideally be performed consecutively to reduce the number of repositioning operations, tool changes, and clamping plate movements.

(4) When multiple operations are performed during a single setup, those that cause the least damage to the workpiece’s rigidity should be scheduled first.

III. Question: What aspects should be considered when determining the workpiece clamping method?

Answer: When determining the positioning reference and clamping scheme, the following three points should be considered:

(1) Strive to unify the references used in design, manufacturing processes, and programming calculations.

(2) Minimize the number of clamping operations as much as possible, aiming to machine all surfaces to be processed after a single positioning.

(3) Avoid clamping methods that require manual adjustments during machine operation.

(4) The fixture should be unobstructed; its positioning and clamping mechanisms must not interfere with toolpaths during machining (e.g., causing collisions). In such cases, clamping with a vise or by adding a base plate and using set screws may be employed.